The phloem of a plant is a vascular tissue that is responsible for distributing the products of photosynthesis, nutrients and hormones to plant tissues and organs. Associated with the phloem are sieve elements and companion cells. Mature sieve cells are enucleate and must rely on physically connected companion cells (via a branched plasmodesmata) to provide many physiological functions. Sieve cells and companion cells together serve to deliver proteins into the phloem. Research has shown that specific mRNA molecules can be found in the plasmodesmata suggesting that there are mechanisms that participate in mRNA transport through the sieve cell-companion cell plasmodesmata connection (Xoconostle-Cazares, B., et al., (1999) Science 283:94-98). Some plant viruses have been shown to be able to establish systemic infections via movement proteins (MP) that have the capacity to interact with the plasmodemata and foster the cell-cell transport of MP and viral nucleic acids. Thus plant viruses have evolved the capacity to utilize existing plant pathways to traffic macromolecules to surrounding cells. Plants appear to have proteins similar to viral movement proteins that function in the transport of nucleic acids from cell to cell. Several plant genes that encode viral movement protein homologs have been identified in rice (elicitor-responsive gene 3, Os-FIERG1 and Os-FIERG2), while one has been identified in corn (novel gene) and one has been identified in Cucurbita maxima (CmPP16) (Xoconostle-Cazares, B., et al., (1999) Science 283:94-98). Interestingly, movement of RNA throughout the plant is postulated by some to explain the phenomena of cosuppression. Thus, understanding plant viral movement protein homologs and how they work will provide mechanisms to control cosuppression and provide mechanisms to engineer plant virus resistance.